The flow of oil or fluid within a vehicle transmission is controlled in a manner to lubricate moving components contained therein and to actuate various subsystems, such as clutches, when needed. A transmission assembly typically includes a sump volume configured to store such fluid, as well as provide desired amounts of the fluid to various components and subsystems within the transmission.
The sump volume must contain a sufficient amount of fluid to maintain an inlet to a hydraulic pump submerged and account for fluid in transit between operational components of the transmission and the main sump volume. That is, for example, fluid directed to the lubrication circuit becomes fluid in transit, i.e., flowing inside the transmission, once the transmission is put in operation thereby reducing the level or amount of fluid contained within the main sump volume. Thus, the initial sump fill must be at a sufficient level to account for the fluid in transit, while maintaining the inlet of the hydraulic pump covered or submerged at all times.
Compared with elevated temperatures, at cold temperatures volume of fluid in transit inside the transmission is very large due to the fluid's viscosity. As temperature increases, fluid volume in transit decreases as viscosity decreases, thereby increasing fluid level within the main sump volume.